The present invention relates to a skid-controlled brake system for use with motor vehicles, comprising a master cylinder pressurizable by a hydraulic force booster. Valve means are provided between the master cylinder and the wheel brakes coupled to the master cylinder, through which valve means pressure fluid can be discharged from the wheel brakes. Pressure fluid discharged from the wheel brakes is reloadable from the pressure chamber of the hydraulic force booster, and the stroke of the brake pedal, during skid control, is limited.
A hydraulic brake system exhibiting these features is known from DE-OS No. 30 40 562.2. This system comprises a hydraulic brake force booster wherein, by way of a pedal-operated brake valve, the pressure of the booster which is proportional to the actuating force is adjustable. The hydraulic force booster is able to actuate a tandem master cylinder coupled to the working chambers of which are wheel brakes. The connections between the working chambers of the tandem master cylinder and the wheel brakes in communication therewith can be disconnected by electromagnetically operable valve means. Moreover, additional valve means are associated with the wheel brakes and are capable of discharging, if need be, pressure fluid from the wheel brakes. As a result thereof, the effective brake pressure in the event of an imminent blocking tendency is decreased so that the vehicle can be adequately reaccelerated.
Pressure fluid as discharged from the wheel brakes during skid control is reloaded from the pressure chamber of the hydraulic force booster. The pressure fluid prevailing in the pressure chamber of the hydraulic force booster is fed, by way of additional valve means, across the cups of the master cylinder piston, into the corresponding working chambers. In these brake systems it is necessary, in order to safeguard a minimum volume in the tandem master cylinder, for the stroke to be limited. For this purpose, when pressure is applied to the working chambers of the tandem master cylinder, pressure is at the same time applied to a so-called positioning sleeve against a spring force in the brake releasing position. This causes the positioning sleeve to come into abutment with a stop of the booster piston and precludes, because of the prevailing effective area conditions, a further displacement of the booster piston and the master cylinder pistons.
In response to the given conditions of the coefficient of friction, in the brake system as described, the case may occur in which the brake pedal, by way of the booster piston, is reset against the actuating force exerted on the brake pedal. Conversely, in low frictional coefficents, at which the blocking limit is already reached at relatively low master cylinder pressures, it is possible for the brake pedal to perform, upon commencement of the skid control, an additional travel in the actuating direction before the stop of the booster piston will get into abutment with the positioning sleeve. An operation of this type is likely to mistakenly convey to the driver the impression that the entire brake system is in a defective condition.
These known brake systems are relatively complex in construction which is due to that the booster piston is guided in the positioning sleeve which, in turn, is slidably disposed in pressure-tight manner on the outer periphery inside the housing. It is, therefore, the object of the present invention to simplify the construction of a hydraulic brake system of the type described and to permit a controlled reset of the brake pedal during skid control. More particularly, the resetting speed is independent of pressure and viscosity.